Amplification and overexpression of HER2 (Neu, ErbB2) is noted in 20-30% of breast cancers. Despite development of targeted therapies such as Herceptin, treatment fails in an alarmingly high proportion of cases, likely due to activation o additional signaling pathways that circumvent treatment. Understanding the role of pathways, such as the E2F transcription factors, in this process will be critical for refining treatment for HER2+ breast cancer. The long term goal of our work is to elucidate the mechanism and progression of breast cancer through molecular and bioinformatic means, with a particular focus on the E2Fs. The immediate objective of this project is to establish the role of the E2Fs in a mouse model of HER2+ breast cancer, determine the mechanism of action and examine how these pathways are utilized in human breast cancer. The central hypothesis of this proposal is that the E2F transcription factors mediate HER2+ breast cancer. Our hypothesis has been predicated on results from a mouse model system and from preliminary results using gene expression from human breast cancer. Specifically, bioinformatic predictions demonstrated differential roles for the E2Fs both in the MMTV-Neu mouse model of HER2+ breast cancer and in human HER2+ tumor samples. Initial results from interbreeding MMTV-Neu mice with E2F knockouts also support this hypothesis. The rationale for the proposed research is that, once it is known how E2Fs regulate HER2+ breast cancer, the downstream targets can be therapeutically targeted, resulting in new and innovative approaches to treatment. We plan to test our central hypothesis and accomplish the objective of this application by investigating the following three specific aims. In the first aim we will test the hypothesis that the E2Fs regulate HER2+ breast cancer in mouse model systems through genetic crosses where we ablate individual E2Fs in the MMTV-Neu transgenic model systems. In the second aim we will elucidate the genetic mechanisms by which E2Fs regulate alterations to tumor latency, growth rate, histology and metastasis through gene expression studies, confirmed through in vitro and in vivo tests. The objective of this aim is to define the genetic mechanisms by which specific E2Fs affect tumorigenesis and to validate these genetic pathways. In the final aim we will assess the role of E2F transcription factors in HER2+ human breast cancer. This proposal is innovative because it will elucidate a novel role for E2F transcription factors in HER2+ breast cancer. This will offer a novel interpretation of the role of E2Fs, as opposed to traditional thinking that places E2Fs simply as cell cycle regulators. This contribution is significant because it will establish a role for the E2Fs in HER2+ breast cancer. Clearly this project will define the role, mechanism and human impact of the E2Fs in HER2+ breast cancer. Once the role for E2Fs in tumor development and progression is determined, the E2Fs will be explored as a clinical biomarker in future work. The implications of uncovering a role for E2Fs in regulation of HER2+ breast cancer are profound, with the potential for development of clinical therapies to enhance survival.